Spiral stability is covered in many standard texts. From Thom...
The lateral stability characteristics of the aeroplane, such as dihedral, cause the lower wing to produce increased lift and to return the aircraft to the wings-level positions. There are two effects in conflict here:
- the directionally stable characteristics (large fin) want to steepen the turn and drop the nose further; and
- the laterally stable characteristics (dihedral) want to level the wings.
If the first effect wins out, i.e. strong directional stability and weak lateral stability (large fin and no dihedral), then the aircraft will tend to bank further into the sideslip, towards the lower wing, with the nose continuing to drop, until the aeroplane is in a spiral dive (all without any input from the pilot). This is called spiral instability.
Most aircraft are designed with only weak positive lateral stability and have a slight tendency to spiral instability. This is preferable to the reverse situation - an effect called Dutch roll.
Further reading...
AvWeb - The Deadly Spiral